1. Field of the Invention
The present invention relates to imaging systems. More specifically, the present invention relates to correlation based imaging target trackers.
2. Description of the Related Art
An autotracker is a device which locates a target in each of a sequence of images and generates commands to maintain a sensor line-of-sight on the target. Correlation trackers generate servo commands based on the position of the target. The tracker measures this position by finding the shift of the input image that maximizes its cross-correlation with a reference image formed by averaging recent past input images.
During a target tracking operation, it often happens that the line-of-sight from the sensor to the target becomes temporarily obscured. For example, in an air-to-ground scenario, a cloud or a building may pass between the sensor and the target temporarily blocking the line-of-sight. One important function of the autotracker is to determine when the target has been obscured, thereby detecting a ‘breaklock’ condition. The tracker is then commanded into a ‘coast mode’ until the target is again visible. In coast mode the sensor is pointed open-loop at the predicted position of the target based on its recent motion. While in coast mode the tracker must continually monitor the input image to determine when the target is again visible so that closed-loop tracking may resume.
Prior attempts to detect breaklock made use of either: 1) a threshold on a statistical average of the residuals after cross-correlation (see, for example, “Developing a Real-Time, Robust, Video Tracker,” K. Plakas, E. Trucco (0-7803-6551-8/00 IEEE) or 2) a threshold on the change in the difference between the input image and the correlation reference map. Unfortunately, these approaches have proven to be too sensitive to certain image disturbances which commonly occur in dynamic tracking situations. This is due to the reliance by these approaches on an assumption that the difference between the input image and reference map pixel values has a Gaussian density. While this may be true in an idealized situation, in practical tracking situations there are several types of image disturbances which cause the Gaussian assumption to falter. One such disturbance is due to a change of the sensor position relative to the tracked target which causes the target's image to change in size and shape. Another is due to uncommanded sensor line-of-sight deviations such as jitter which cause image blurring. These image disturbances will cause the existing breaklock detection methods to falsely declare a breaklock condition when continued tracking is still feasible.
Hence, a need exists in the art for an improved system or method for detecting breaklock for correlation trackers.